When agitating film, bubbles will form. This only matters if the bubbles become trapped in the spiral next to the film and slow down the development process where the bubble rests.
A bubbles problem looks like little beads of thinness in the negative along one edge of the film; when printed or scanned, they will look like little dark patches in the image. They're generally only visible in 120 film where the image goes to the edge of the film whereas with 35mm, the bubble will tend to hang around the sprocket holes where there is no image.
To prevent bubbles, you should put one one or two drops only of Photoflo in the diluted developer just before you use it; this will allow the bubbles to slip through the spiral instead of being trapped. If you put too much Photoflo in, it will foam and cause even worse problems.
If you suspect a bubble problem, you can load some old film into a spiral, fill your tank with plain water, agitate it for a little then take the lid off. The bubbles will be visible if they're present. If you have problematic tap water, no amount of rapping the tank will dislodge the bubbles without a surfactant.
This problem does not occur with rotary agitation systems; Photoflo should never be used in a Jobo or similar system.
See example image.
Back to Analogue Photography and Film FAQ